Why is it easy to cause water hammer if a "check valve" is not installed at the pump outlet?

First, let's take a look at the consequences of not installing a check valve on the water pump:

The sudden changes in the water flow inside the pump outlet pipeline, such as opening, closing, and stopping the pump, cause a sudden change in the flow velocity inside the pipeline, resulting in a change in momentum per unit time. This inevitably generates corresponding inertial forces, leading to alternating changes in pressure inside the pipeline. The phenomenon of water flow velocity and pressure changing with time and position is called water hammer (or water hammer).

Pump station water hammer includes starting water hammer, closing valve water hammer, and stopping pump water hammer (caused by sudden power outage and other reasons). The first two types of water hammers will not cause any safety hazards to the unit under normal operating procedures. The water hammer pressure caused by the latter is often very high, leading to accidents.

Therefore, the purpose of studying and calculating water hammer is to:

① Develop protective measures when the highest water hammer pressure causes damage to pipelines and units;

② Develop the minimum water hammer pressure that causes unacceptable negative pressure inside the pipeline, as well as protective measures in case of pipeline damage;

③ Prevent damage caused by unit reversal.

In the pumping system, the characteristics of the pump serve as the boundary condition at the starting end of the pipeline. Now, let's analyze the water hammer process (also known as hydraulic transition process) when the pump outlet is not equipped with a check valve and the pipeline outlet is not equipped with a flap valve. When the pump stops in an accident, the pump loses its driving force, and the gate valve on the outlet side of the pump cannot be closed in a timely manner, resulting in water backflow in the pipeline.

1. Water pump working condition

After the pump loses power, the speed of the pump drops sharply, and the water flow in the pump and outlet pipeline continues to move in the original direction due to inertia. However, its flow rate rapidly decreases, and the pressure drops until the water flow stops flowing forward and the flow rate is zero. At this moment, the water pump is still in forward rotation, and the water flow is in forward flow (the movement of water from the pump towards the outlet pool is called forward flow), which is called the pump operating condition.

2. Braking conditions

In a transient stationary water body with zero flow rate in the outlet pipe, due to the action of the static head of the outlet water tank, a reverse water flow is generated in the outlet pipe from the outlet water tank to the water pump. The reverse water flow acts as a brake on the impeller of the water pump that is still in forward rotation, forcing the rotor speed of the unit to accelerate and decrease until the speed reaches zero. At this moment, due to the resistance of the forward rotating impeller against the counter flow, the pump outlet pressure gradually increases. The transient condition of the water pump at this moment is called the braking condition.

3. Hydraulic turbine operating conditions

As the reverse flow rate increases, the speed of the water pump starts to reverse from zero and rapidly rises. At the same time, the centrifugal force of the rotating impeller on the water flow also increases, hindering the reverse flow. This resistance increases with the acceleration of impeller reversal, causing a rapid increase in pressure after the pump, reaching its maximum value at a certain moment, and the corresponding reverse speed also reaches its maximum. At the same time as the resistance increases, it also resists the further increase of reverse flow rate, and gradually decreases after reaching a certain maximum value. The energy acting on the impeller also decreases accordingly, causing the reverse speed to gradually decrease until under the action of a stable static water head, the torque of the water flow acting on the impeller is balanced with the resistance torque of the rotating part of the unit. The unit works like a water turbine at a constant reverse flow rate and speed without any load, and the water hammer pressure disappears. This transient operating condition is called the turbine operating condition.

The above are the consequences of not installing a check valve on the water pump. At the same time, the pressure change after the pump will shoot in the form of water hammer waves along the outlet pipeline towards the outlet pool, and then reflect back from the outlet pool, forming a complex water hammer phenomenon in the outlet pipeline. The check valve product of the waterproof hammer phenomenon can be selected as a waterproof hammer check valve.

More information can be found on the official website of Shenzhen Hongjie Water Technology Co., Ltd. www.czhengwei.net. If you need it, you can call the company's hotline for free at 180 3800 0078, and we will be happy to serve you.